Not sure if this is the right Forum for help with this horrible problem, Just all of a sudden started happening in the middle of a project. Sonar - and computer stops seeing my interface - Focusrite Scarlett 18i20 (USB, Win 7) - If I re-boot, things are OK using e.g. Windows Media Player. But if I bring up Sonar and play a project there is no audio - even in the channel gauges with audio in them already. And the Scarlett MixControl App says "No Hardware Connected" - although before I instantiate Sonar (SPLAT) it sees the interface and apparently works fine. When I press Play on Sonar there is significant latency to start. I have restore Win 7 to a previous set point, set the Audio config file to default and this keeps happening. I haven't a clue what to do. Any help would be immensely appreciated. Rom 

Try uninstalling the drivers for your Scarlett and after disconnecting it reboot. Then reinstall it using the method from its manual. Make sure the drivers are for Windows 7. Also if you can avoid using Windows media player using that interface. I have my system set to use the on board sound chip as the default for Windows, Sonar uses my Roland interface. 
 
You list a Saffire in the sig and the Scarlett in the post. Which is it?
 
 

Do you keep MixControl open?  I had this exact same problem and found after much frustration and extensive testing that it was caused by leaving MixControl open all the time.  Now, I open MixControl as needed and then immediately close it after making my changes and haven't seen the issue since.
 

Thx - will try those suggestions - It's a Scarlett - will fix the sig

I use ASIO drivers for my audio interface, and because of that, I only have Sonar set to use it - setting the Windows Default Audio Device to be the HDMI output to an HDTV I use as a computer display monitor.
 
Doing the above means I never have conflict between multiple applications (including Windows) attempting to access the audio interface through its ASIO drivers.
 
Hope that helps, 
 
Bob Bone
 

robert_e_bone
I use ASIO drivers for my audio interface, and because of that, I only have Sonar set to use it - setting the Windows Default Audio Device to be the HDMI output to an HDTV I use as a computer display monitor.
 
Doing the above means I never have conflict between multiple applications (including Windows) attempting to access the audio interface through its ASIO drivers.
 
Hope that helps, 
 
Bob Bone
 

Bob that explanation is extremely helpful. Its what I should have added but failed to.  

Dave76's suggestion fixed it. No problems with Sonar or even WMP as long as I don't have MixControl open when Sonar runs. So that seems to be the way to do it. I can set the routing in MixControl (for monitoring) and save the configuration to the Scarlett Hardware, at a separate time than Sonar is running.
 
What I don't understand is why this just started happening. I've been using this Set-up since around Thanksgiving.
 
But I'm back in business - thx for your help - Ron

NeuroRon
What I don't understand is why this just started happening. I've been using this Set-up since around Thanksgiving.
 

Have you changed anything in terms of plug-ins, features, etc. that you commonly use?  In my case, this only ever happened with really strange combos of things.  I could have a project with a ton of tracks, plug-ins, virtual instruments, etc. and never have an issue and then I'd have a two track project and it would happen all the time.  
 
I should also mention that, in my testing, it was not a SONAR issue.  I set up some other DAWs with the exact same test scenario and the same thing happened.  Buggy driver, I guess.  Too bad that Focusrite hasn't bothered to officially release a new driver for the 18i20 in two years.
 
In any case, glad to be a help and keeping MixControl closed is fortunately not too much of a hassle.